Influence of liquid phase lead borate glass on dielectric response of lead iron niobate

Journal of Alloys and Compounds 587 (2014) 26–31

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Influence of liquid phase lead borate glass on dielectric response of lead iron niobate Tanveer Quazi a,⇑, Kamal Singh b, Shahin Sayyed b, S. Acharya b a b

Anjuman College of Engineering & Technology, Nagpur 440001, M.S., India Department of Physics, Rashtrasant Tukdoji Maharaj Nagpur University, Nagpur 440033, M.S., India

a r t i c l e

i n f o

Article history: Received 30 May 2013 Received in revised form 14 October 2013 Accepted 15 October 2013 Available online 26 October 2013 Keywords: Ferroelectrics Sintering Dielectric response Scanning electron microscopy (SEM)

a b s t r a c t h   i þ5 Lead iron niobate Pb Feþ3 1=2 Nb1=2 O3 (PFN) is well recognized lead-based complex-double perovskite [A(BIBII)O3] ferroelectrics. In the present work, the influence of liquid phase of lead borate glass 4PbO-B2O3 (LB) on sinterability and dielectric behaviours of PFN are systematically investigated. The PFN powder was synthesized by combustion route (SHS synthesis) and LB-glass by conventional quenching technique, separately. The compositions of PFN and LB-glass (0–3 wt%) were prepared by ball milling. The crystalline structure of as-synthesized PFN, PFN–LB and amorphous nature of LB-glass are confirmed by XRD and well fitted by Rietveld refinement with monoclinic structure of ‘Cm’ space group. Microstructure was examined by SEM and TEM; indicates nanoscale dimensions of as-synthesized PFN. The green pellets of PFN and PFN–LB were sintered at various sintering conditions. Liquid phase sintering by LBglass in PFN is found to strongly influence on the morphology, compactness and dielectric behaviour of nanostructure PFN. Semiquantitative correlation between microstructure and dielectric behaviour was established. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction h   i þ5 Lead iron niobate Pb Feþ3 1=2 Nb1=2 O3 (PFN) is belongs to leadbased complex-double perovskite [A(BIBII)O3] ferroelectric family. It was found most attractive due to their exceptionally high dielectric constant for making multilayer ceramic capacitor [1–3]. Suitability of PFN to yield cost effective multilayer capacitors has been realized by lowering the sintering temperature (<1000 °C); it makes to use non-noble inexpensive materials (viz., 70:30 Ag: Pd, etc.) as electrode for multilayer capacitor [4–6]. A diffuse phase transition (DPT) with a non-relaxor type behaviour has been observed in PFN [7]. This makes the research on PFN-system more attentive and attempts have been mostly focused on development of high quality PFN-based system having low sintering temperature with enhancing dielectric constant, less loss and controlling lead loss during processing [8–12]. Optimizing processing conditions to obtain high quality PFN for device application is a major challenge. Recent literature reveals that the liquid phase sintering (LPS) of ceramic using B2O3, Bi2O3, LiF, etc as sintering aids has been found to be a highly effective way to decrease sintering temperature with improving dielectric constant and minimizing lead loss during ⇑ Corresponding author. Tel.: +91 09326449300. E-mail address: [email protected] (T. Quazi). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.114

sintering [13–16]. In general, liquid phase sintering in ceramics has been observed to induce complete densification at much lower temperature [17–19,15]. In addition to that, enhancement in dielectric and piezoelectric properties of ceramics has been reported in very few cases by liquid phase sintering [20–27]. A systematic review of recent literatures on use of glass as sintering aids to improve sinterability and dielectric behaviour in ferroelectric ceramics is given in Supplementary data file-1. Amongst the available glasses system, PbO-based glasses have been observed to have lowest softening temperature, around 500 °C, high ionic polarizability and low dielectric losses and thus are more appropriate for liquid phase sintering [16,28]. With this motivation, in the present work, PbO-based glass are selected as sintering aids in the PFN double perovskite system. Structure is confirmed by X-ray diffraction study. Microstructures are examined by SEM and TEM. Sinterability of the PFN–LB system is systematically investigated by using liquid phase sintering and results are compared with pure PFN system. Effects of liquid phase sintering on dielectric behaviour are investigated. Semiquantitative correlation between microstructure and dielectric behaviour is established. 2. Experimental The initial ingredients used in this work were Pb(NO3)2 (99% Merck), Fe(NO3)39H2O (99% Merck) and Nb2O5 (99.5 Sigma–Aldrich) as cation precursors and urea, CO(NH2)2(99% Merck) as a fuel. The precursor were initially mixed in 50 ml distilled

T. Quazi et al. / Journal of Alloys and Compounds 587 (2014) 26–31

27

water and stirred for 15 min to obtain a homogeneous mixture. The homogeneous solution was heated at 250 °C (optimized temperature), the liquid frothed for a while, followed by ignition with evolution of gases and combustion occurred. To complete the combustion reaction, the reactants were further heated at 550 °C for 10 min in muffle furnace; reddish brown product of PFN was obtained. The product was grounded at room temperature in a mortar. The LB-glasses were prepared by following earlier reported conventional quenching technique; ingredients B2O3 and PbO (Merck) were used for synthesis [15]. LB-glass in different weight percentages (0.0 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 wt%) were dispersed in PFN to form different compositions. The compositions were mixed in presence of acetone with the help of ball mill (Pulverisette-6 Fritz, Germany). During ball milling, the milling rate was maintained at 600 rpm per hour. The powder was compacted using pressure of 5 ton/cm2. The compacted pellets were sintered at temperature more than that of glass softening temperature. The density of the sintered pellet was determined using Archimedes principle [29] relative density was calculated. Table 1 lists the nomenclature used in the present work to represent various compositions of PFN, LB and their sintering conditions. When the LB-glass percentage in PFN was increased above 3 wt%, then sintered pellet was found to melt, above 700 °C. This is due to softening temperature of LB-glass around 500 °C. Hence, maximum upto 3 wt% LB is added in PFN as sintering aids in the present work. The crystal structure was confirmed by X-ray powder diffraction (XRD) using Cu Ka1 radiation, with the help of PANalytical X-ray powder diffractometer at 40 kV and 30 mA. Microstructure and surface morphology was examined by Transmission Electron Microscopy (TEM) CM200 TEM and Scanning electron microscope (JEOL: JSM-6380 Analytical SEM) equipped with an electronic probe analyzer system (Accelerating voltage 30 kV). For dielectric study, samples were coated with platinum for good ohmic contact. The dielectric constant (er) and dissipation factor (tan d) were measured at different frequencies from 10 Hz to 1000 kHz in 25– 200 °C temperature range by using computer controlled HP-4192A LF impedance analyzer. The temperature during the measurements was controlled with an accuracy of ±1 °C using Eurotherm temperature controller.

3. Results

Fig. 1. XRD patterns of (a) PFN (b) 4PbO:B2O3 glass and (c) PFN-3-LB-83.

The XRD patterns of pure PFN, 4PbO:B2O3 (40:60 wt%) LB-glass and PFN-3-LB-83 systems are shown in Fig. 1(a–c), respectively. XRD patterns of PFN (Fig. 1a) is well matched with the JCPDS data (file no. 0.032-0522) confirm the formation of single phase with monoclinic structure of PFN in ‘‘Cm’’ space group symmetry. The observed XRD peaks are well matched with the lines of perovskite phase of PFN. No additional impurity phases are found. The amorphous nature of the LB-glass is clearly depicted by Fig. 1b. XRD pattern of PFN-3-LB-83 reveals the presence of sharp crystalline peaks of PFN (Fig. 1c). The XRD data of both the samples (pure PFN and LB glass-dispersed PFN) are well fitted with Rietveld by ‘‘Cm’’ space group symmetry (shown in Fig. 2). TEM image of the as-synthesis PFN shows (Fig. 3) oval shape of particles having diameter around 50 nm. The diffusive characteristics of PFN as viewed in the inset of Fig. 3 demonstrate the random distribution of Fe3+ and Nb5+. It is a clear evidence of existence of compositions fluctuation of the Fe3+ and Nb5+, which leads to formation of many microscopic regions on the B-sites. The same trends of results have earlier been reported by Cross [30]. SEM images of sintered pellets of PFN-0-LB-83 and PFN-2-LB-83 samples are shown in Fig. 4(a and b), respectively. Both the images Table 1 List of Compositions of PFN–LB with its nomenclature PFN–LB. Composition

PFN+0 wt%LB PFN+1 wt%LB PFN+2 wt%LB PFN+3 wt%LB PFN+0 wt%LB PFN+1 wt%LB PFN+2 wt%LB PFN+3 wt%LB

Sintering conditions

Nomenclature

Sintering temperature (°C)

Sintering time (h)

700 700 700 700 700 800 800 800 800 800

2 3 3 3 3 2 3 3 3 3

PFN-0-LB-72 PFN-0-LB-73 PFN-1-LB-73 PFN-2-LB-73 PFN-3-LB-73 PFN-0-LB-82 PFN-0-LB-83 PFN-1-LB-83 PFN-2-LB-83 PFN-3-LB-83

reveal highly compacted structure with well defined grain and grain boundaries. However, glass deposition on the grain boundary can be clearly detected in SEM image of PFN-2-LB-83. Grain size was determined by manual image analysis, matching digital pixel scale across each grain. The average grain size of PFN-0-LB-83 and PFN-2-LB-83 is obtained to be 1.9 lm and 2.4 lm, respectively. This clearly shows that under the same sintering conditions grain size of PFN-2-LB-83 is more than PFN without LB. Relative density of both the system are determined and found more in PFN-2-LB83. Same trends of variation of grain size and relative density are observed in almost all samples (tabulated in Tables 2 and 4). However, if the LB concentration increases above 2 wt%, relative density and grain size both are found to decrease. The observed enhancement of sinterability of PFN by LB is understood on the basis of liquid phase sintering (LPS) process. The glass softening temperature of LB is around 500 °C, at which LB starts melting. In the present work sintering were performed above the glass softening temperature (at 700 °C and 800 °C), at which LB melt and liquid form of LB move around the grain boundaries of PFN grain due to capillary action. In capillary action, the capillary force arises due to surface tension at the interface (liquid–solid–vapour) leads the flow of liquid phase. On the same line, in the present case flow of LB through grain boundary fills the pores of PFN which leads to densification and grains-growth [31–36]. Thus increase in relative density using LB clearly shows that in PFN–LB sintering process is dominantly due to the liquid phase sintering (LPS) rather than solid-state sintering. However with an increase LB concentration above 2 wt%, decrease of sinterability is due to bridging of particle by glass materials, which can disintegrate the solid particles into smaller grains at the liquid-solid interface. The PFN-2-LB-83 is found to optimize glass percentage in the liquid phase sintering. The temperature profile of dielectric constant at 1 kHz frequency of PFN–LB samples sintered at 700 and 800 °C are displayed in Figs. 5 and 6, respectively. Dielectric anomaly around 100 °C can be clearly observed for both the samples. The dielectric maxima are

28

T. Quazi et al. / Journal of Alloys and Compounds 587 (2014) 26–31

(a)

(b)

Fig. 4. Scanning electron microphotograph of sintered sample (a) PFN-0-LB-83 (pure PFN) and (b) PFN-2-LB-83 (PFN-2 wt% LB glass) sintered at 800 °C for 3 h.

Fig. 2. Rietveld refinement of (a) PFN and (b) PFN-3-LB-83.

The addition of LB in PFN is found to shift the transition temperature towards lower side. Same types of results have earlier been reported in the literature [40–44]. Comparison of dielectric behaviour of 700 °C and 800 °C sintered PFN–LB (Fig. 7); indicate that LB influence on dielectric behaviour of PFN vary with sintering temperature. For 700 °C sintered sample, dielectric constant increases and loss tan d followed the natural decrease with increase of LB glass content; however for 800 °C sintered sample, the dielectric constant enhanced, though loss factor (tan d) decreases drastically as the glass concentration vary from 1 to 2 wt%; and thereafter it decreases for higher percentage of glass. The trends of variation of dielectric constant with grain size and sinterability are tabulated in Table 3. The trends exhibits that dielectric constant exclusively depends upon microstructure. Frequency dependent dielectric behaviour of pure and LB glass-PFN (see Figs. 8 and 9, respectively) have not shown any significant difference in dielectric constant and tan d. It reveals that addition of LB in PFN is not affecting on relaxing behaviour of PFN. To get more in depth investigation on influence of macroscopic parameters of materials on dielectric behaviour, the macroscopic parameters are qualitatively correlated with dielectric constant. 4. Discussion

Fig. 3. TEM of as prepared (before sintering) PFN prepared at 500 °C.

shifted to lower temperature side from 118 °C to 90 °C with increasing LB percentage from 0 to 3 wt%. Inset of Figs. 5 and 6 clearly demonstrate the shifting. The dielectric anomaly can be correlated with monoclinic to cubic phase transition temperature (DPT) of PFN which has been reported at 114 °C [37–39].

For semiquantitative understanding of the correlation of dielectric constant with microstructure of materials, the dielectric constant values should be first corrected for porosity as well as the second phase present. For the correction of the porosity effect on the dielectric constant (er max), there are two quantitative approaches given in the literature on the basis of porosity percentage Rushman and

29

T. Quazi et al. / Journal of Alloys and Compounds 587 (2014) 26–31 Table 2 Density correction on observed dielectric constant, corrected dielectric constant, transition temperature and grain size for pure PFN. Sr. no 1 2 3 4

Sample name PFN-0-LB-72 PFN-0-LB-73 PFN-0-LB-82 PFN-0-LB-83

Tsinter (°C) 700 700 800 800

tsinter (h) 2 3 2 3

Density (%) 88 80 90 92

Porosity (%) 12 20 10 8

Table 3 Variation of dielectric constant with the density and grain size of the materials [(") increase, (;) decrease symbol]. Density

Grain size

Dielectric constant (er)

Result of sample

" ;

" ;

" ;

" ;

; "

; ;

Case of PFN-2-LB-83 Case of PFN-1-LB-73, PFN-2-LB-73, PFN-3LB-73, PFN-1-LB-83, PFN-3-LB-83 – –

Strivens equation and Wiener’s equation [45]. Rushman and Strivens equation (1) is found most suitable to correct dielectric constant for porosity less than 35%.

erðCorrectedÞ ¼

erðObservedÞ  ð2 þ V 2 Þ

ð1Þ

2ð1  V 2 Þ

where V2, is the volume fraction of porosity in the sample and

er(observed) is the observed dielectric constant. The above equation was experimentally verified upto 35% porosity (V2 = 0.35). The samples containing more than 35% porosity, Wiener’s equation (2) has been used for correction of dielectric constant [45].

ðerðGrainÞ  erðTotalÞ Þ  ðerðPoreÞ þ 2erðGrainÞ Þ

Vol fraction of porosity (V2)

er(observed) at

er(corrected), at

1 kHz

1 kHz

0.12 0.20 0.10 0.08

13,500 11,348 14,000 15,500

14,420 12,766 14,777 16,173

ð2Þ

where er(Grain), er(Pore) and er(Total) are the dielectric constant of the PFN grain, pore and total, respectively. In the present work, as porosity of all PFN samples are less than 35% hence we have used Rushman and Strivens equation (1) for correction of dielectric constant. In the corrected values of pure-PFN and LB–PFN are tabulated in Tables 2 and 4, respectively. For both the samples i.e. pure-PFN and LB–PFN in all sintering conditions, the corrected value of dielectric constant (er max) shows a monotonic increase with the increase of grain size from 1.9 to 2.4 lm. The dielectric-grain size dependency has been earlier observed for PMN and other lead-based relaxors. This dielectric dependency has been explained by considering (i) micro dipole-dipole co-operative interaction between superparaelectric regions and (ii) a low polarizable phase boundary [46]. Similarly, dielectric dependency on grain size has been observed in lead borate added BaTiO3 [47–49].

Tc (°C)

Grain size (lm)

1.2 0.06 6 0.4

112 115 110 118

1.2 1 1 1.9

Effect of sintering aid (LB) on dielectric constant of PFN can be estimated by using logarithmic mixing rule given by Linchtenecker [50]. The mathematical equation for logarithmic mixing rule, which is used to find effective dielectric constant, is given below.

loge erðPFNþGlassÞ ¼ mðPFNÞ loge erðPFNÞ þ mðGlassÞ loge erðGlassÞ

ð3Þ

where vGlass, vPFN, er Glass and er PFN, respectively are volume fraction and dielectric constant of the LB-glass, PFN materials. For this calculation, the weight% ratio is converted into volume fraction as shown in Table 4. The effective dielectric constant of LB–PFN for different percentage of LB is calculated on substituting values of volume fraction and dielectric constant (er max) at 1 kHz of PFN and LB-glass in Eq. (3). The calculated value of dielectric constant (er Calculated) of all the LB-glass dispersed PFN is given in Table 4. By comparing er(Calculated) with the er(Corrected), the contribution of LB phase (er(Glass)) on dielectric constant of PFN is obtained 20. For more in depth understanding of the effect of sintering aids on the microstructure and their correlation with dielectric constant, LB influence on grain boundary thickness of PFN is needed to be estimated. The grain boundary thickness of LB–PFN can be determined by the series mixing theory of Wand and Schulze [51]. This theory uses the following equation:

D

¼ ðerðGrainÞ  erðPoreÞ Þ  V 2

tan dmax

erðsÞ

¼

Dg

erðgÞ

þ

Dgb

ð4Þ

erðgbÞ

where er(s) is the dielectric constant of the LB–PFN, er(g) is the dielectric constant of the pure-PFN grain, er(gb) is the dielectric constant of the LB-glass in the grain boundary, D is the thickness of the sample, Dg is the thickness of grains (i.e. grain size) of the PFN and Dgb is the thickness of the LB-glass layer at the grain boundary. Thus, the dielectric constant of the grain boundary is taken as the dielectric constant of LB-glass from Eq. (4). As grain boundary thickness is much smaller as compared to grain size; D  Dg and Eq. (4) reduces to

1

erðsÞ

¼

1

erðgÞ

þ

1 RerðgbÞ

ð5Þ

where R = Dg/Dgb i.e. the thickness ratio of the grain to the grain boundary layer. In the present study er(g) (dielectric constant of the grain) is higher than er(gb) (dielectric constant of the LB-glass in the grain boundary). The er(gb) for LB-glass is 20 and not much

Table 4 Volume fraction, density correction on dielectric constant, calculated dielectric constant, transition temperature, grain size and thickness of grain boundary for PFN–LB glass sample. Sample code PFN-1-LB-73 PFN-2-LB-73 PFN-3-LB-73 PFN-1-LB-83 PFN-2-LB-83 PFN-3-LB-83

Density (%)

Vol, fraction of porosity (V2)

Vol, fraction of pure PFN (vPFN)

Vol, fraction of LBglass (vGlass)

er(Observed)

er(Corrected)

er(Calculated) at

at 1 kHz

at 1 kHz

(1 kHz)

Tc (°C)

Grain size (lm)

Dgb (nm)

85 89 83 92 95 94

0.15 0.11 0.17 0.08 0.05 0.06

0.99 0.98 0.97 0.99 0.98 0.97

0.01 0.02 0.03 0.01 0.02 0.03

15,876 16,334 16,478 16,778 21,724 15,246

20078.47 19362.21 21540.51 18966.43 23439.05 16705.72

14413.34 13454.41 12315.04 15223.76 17792.08 11420.96

112 107 90 107 105 90

1.5 1.7 2.0 1.8 2.4 2.2

0.95 1.14 1.13 1.24 1.75 1.83

30

T. Quazi et al. / Journal of Alloys and Compounds 587 (2014) 26–31

0

700

14000

800

20000

12000

0wt% 1wt% 2wt% 3wt%

10000 8000

εr

Dielectric constant (εr )

22000

Frequency 1kHz

700 C

16000

18000

16000

Dielectric loss (tan δ )

0.3

0.2

14000

0

700 C Linear Fit

120 115

0.06

110 105

0.05

100 95 90

tanδ

o

Transition temperature ( C)

6000

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.1

Concentration of glass (wt%)

0.03

0.02

0.0 20

40

60

80

100

120 140

160

180

200

220

0

Temperature ( C)

0.01 0.5

Fig. 5. Variation of dielectric constant and loss with the temperature for LB-glass added PFN (1–3 wt%) at 1 kHz frequency for the pellets sintered at 700 °C (inset: transition temperature versus concentration of LB-glass).

800 C

12000 9000 6000 0

800 C Linear Fit

120 o

Transition temperature ( C)

2.0

2.5

3.0

3.5

Fig. 7. Dielectric constant and dielectric loss for different concentration of glass sample sintered at 700 °C and 800 °C, for 1 kHz frequency.

115 110

0

0wt%

sinter at 700 C Dielectric constant( εr)max

15000

1wt%

16000

2wt% 3wt% 15000

14000

105

0.2

100 95 90 0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.1

Concentration of glass (wt%)

0.0 20

40

60

80 100 120 140 160 180 200 220

Temperature (0C) Fig. 6. Variation of dielectric constant and loss with the temperature for LB-glass added PFN (1–3 wt%) at 1 kHz frequency for the pellets sintered at 800 °C (inset: transition temperature versus concentration of LB-glass).

temperature dependent, whereas er(s) is taken as er(corrected). For er(g), consider a reported dielectric constant of PFN at Tc for this calculation [52]. All these values are used to estimate thickness of grain boundary by using Eq. (5). Thickness of LB layer can be predicted from the grain boundary thickness. LB layer thickness of all the

0.06

Dielectric loss(tanδ)

Dielectric loss (tanδ)

1.5

Concentration of lead glass(wt%)

0wt% 1wt% 2wt% 3wt%

18000

0.3

1.0

Frequency 1kHz

0

21000

Dielectric constant (εr)

0.04

0.04

0.02

0.00 0.0

2.0x10

5

4.0x10

5

6.0x10

5

8.0x10

5

1.0x10

6

Frequency (Hz) Fig. 8. Frequency dependence of dielectric constant maxima and loss for different concentrations of LB-glass (0–3 wt%) sintered at 700 °C.

T. Quazi et al. / Journal of Alloys and Compounds 587 (2014) 26–31

Dielectric constant(εr)max

References

0

24000

sinter at 800 C

0wt% 1wt% 2wt%

21000

3wt%

18000

15000

Dielectric loss(tanδ)

0.06

0.04

0.02

0.00 0.0

2.0x10

5

4.0x10

31

5

6.0x10

5

8.0x10

5

1.0x10

6

Frequency (Hz) Fig. 9. Frequency dependence of dielectric constant maxima and loss for different concentrations of LB-glass (0–3 wt%) sintered at 800 °C.

samples is obtained to be in 1–2 nm range; is tabulated in Table 4. However the presence of nanometer range layer of glass phase (SEM, Fig. 4b) around each grain may be the reason for the increase of dielectric constant from 15,500 to 21,724 of PFN-2-LB-83 sample as shown in Fig. 7. 5. Conclusion The enhancements of sinterability with the addition of LB in PFN are explained by liquid phase synthesis process. Liquid phase sintering by LB-glass in PFN is found strongly influencing on the compactness, morphology and dielectric behaviour of nanostructure PFN material. The transition temperature for dielectric maxima are also found to be shifted to lower temperature side from 118 to 90 °C due to LB dispersion in PFN. The increase in densities of PFN–LB over pure PFN is also confirmed by SEM study. The semiquantitative correlation between microstructure and dielectric behaviour are established. Acknowledgements Authors are thankful to DRDO, New Delhi (INDIA) for providing financial assistance to carry out this study. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2013. 10.114.

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